attached you find the output stage from my Perpetual Technologies P3-A with crystal CS 4396 DA convertor (voltage output, balanced).
Besides trying to understand this (especially the second opamp) I would like to convert this to something with a passive filter, x-bosoz with ccs (like Henrik showed in another thread) and keep everything balanced.

Just for fun I was looking what filter frequencies are in this standard output and hereīs where the problem starts.

If I look at the first low-pass (1k, 5n6//1k) I īm already 2.6dB down at 20.000Hz, The second stage (499R, 1n5) ads another 0,7dB to this so Iīm down over 3dB at 20000Hz. Since this certainly isnīt the case I must be making a mistake somewhere in this (very simple) calculation